WIP: create feature to dynamically set readpreference on a call-by-ca…

grails-datastore-gorm-mongodb/src/main/groovy/grails/mongodb/MongoEntity.groovy

@@ -216,6 +216,10 @@ trait MongoEntity<D> implements GormEntity<D>, DynamicAttributes {
         currentMongoStaticApi().useDatabase(databaseName)
     }
 
+    static <T> T withReadPreference(String readPreferenceString, Closure<T> callable) {
+        currentMongoStaticApi().withReadPreference(readPreferenceString, callable)
+    }
+
     /**
      * Counts the number of hits
      * @param query The query

grails-datastore-gorm-mongodb/src/main/groovy/grails/mongodb/api/MongoStaticOperations.groovy

@@ -76,6 +76,14 @@ interface MongoStaticOperations<D> extends GormStaticOperations<D> {
      */
     public <T> T withCollection(String collectionName, Closure<T> callable)
 
+    /**
+     * Use the given read preference for the scope of the closure call
+     * @param readPreferenceString The read preference as string (primary, secondaryPreferred, etc)
+     * @param callable The callable
+     * @return The result of the closure
+     */
+    public <T> T withReadPreference(String readPreferenceString, Closure<T> callable)
+
     /**
      * Use the given collection for this entity for the scope of the session
      *

grails-datastore-gorm-mongodb/src/main/groovy/org/grails/datastore/gorm/mongo/api/MongoStaticApi.groovy

@@ -132,6 +132,19 @@ class MongoStaticApi<D> extends GormStaticApi<D> implements MongoAllOperations<D
         }
     }
 
+    @Override
+    def <T> T withReadPreference(String readPreferenceString, Closure<T> callable) {
+        withSession { AbstractMongoSession session ->
+            final previous = session.getReadPreference()
+            try {
+                session.setReadPreference(ReadPreference.valueOf(readPreferenceString))
+                return callable.call()
+            } finally {
+                session.setReadPreference(previous)
+            }
+        }
+    }
+
     @Override
     String useCollection(String collectionName) {
         withSession { AbstractMongoSession session ->

grails-datastore-gorm-mongodb/src/main/groovy/org/grails/datastore/mapping/mongo/AbstractMongoSession.java

@@ -14,6 +14,7 @@
  */
 package org.grails.datastore.mapping.mongo;
 
+import com.mongodb.ReadPreference;
 import com.mongodb.WriteConcern;
 import com.mongodb.client.MongoClient;
 import org.bson.Document;
@@ -45,6 +46,7 @@ public abstract class AbstractMongoSession extends AbstractSession<MongoClient>
     protected final String defaultDatabase;
     protected MongoDatastore mongoDatastore;
     protected WriteConcern writeConcern = null;
+    protected ReadPreference readPreference = null;
     protected boolean errorOccured = false;
     protected Map<PersistentEntity, String> mongoCollections = new ConcurrentHashMap<PersistentEntity, String>();
     protected Map<PersistentEntity, String> mongoDatabases = new ConcurrentHashMap<PersistentEntity, String>();
@@ -135,6 +137,14 @@ private WriteConcern getDeclaredWriteConcern(WriteConcern defaultConcern, Persis
         return writeConcern;
     }
 
+    public void setReadPreference(ReadPreference readPreference) {
+        this.readPreference = readPreference;
+    }
+
+    public ReadPreference getReadPreference() {
+        return readPreference;
+    }
+
     public MongoClient getNativeInterface() {
         return ((MongoDatastore)getDatastore()).getMongoClient();
     }
@@ -182,10 +192,16 @@ public com.mongodb.client.MongoCollection<Document> getCollection(PersistentEnti
         if(entity.isRoot()) {
             final String database = getDatabase(entity);
             final String collectionName = getCollectionName(entity);
-            return getNativeInterface()
+            com.mongodb.client.MongoCollection<Document> collection = getNativeInterface()
                     .getDatabase(database)
                     .getCollection(collectionName)
                     .withCodecRegistry(getDatastore().getCodecRegistry());
+            final ReadPreference readPreference = getReadPreference();
+            if (readPreference != null) {
+                return collection.withReadPreference(readPreference);
+            } else {
+                return collection;
+            }
         }
         else {
             final PersistentEntity root = entity.getRootEntity();

grails-datastore-gorm-mongodb/src/test/groovy/org/grails/datastore/gorm/mongo/ReadPreferenceSpec.groovy

@@ -0,0 +1,42 @@
+package org.grails.datastore.gorm.mongo
+
+import grails.gorm.annotation.Entity
+import grails.mongodb.MongoEntity
+import com.mongodb.ReadPreference
+import grails.gorm.tests.GormDatastoreSpec
+import org.junit.platform.commons.logging.LoggerFactory
+
+class ReadPreferenceSpec extends GormDatastoreSpec {
+    List getDomainClasses() {
+        [PotatoFarm]
+    }
+
+    void "Gets read preference from collection correctly"() {
+        when: "Read read concern by default"
+        def rp = PotatoFarm.getCollection().getReadPreference()
+
+        then: "It is primary"
+        rp == ReadPreference.primary()
+
+        when: "Ask for secondary"
+        def rp2 = PotatoFarm.withReadPreference("secondary") {
+            PotatoFarm.getCollection().getReadPreference()
+        }
+
+        then: "It is secondary"
+        rp2 == ReadPreference.secondary()
+
+        when: "After asking for secondary"
+        def rp3 = PotatoFarm.getCollection().getReadPreference()
+
+        then: "It returns to primary"
+        rp3 == ReadPreference.primary()
+    }
+}
+
+@Entity
+class PotatoFarm implements MongoEntity<PotatoFarm> {
+    String id
+    String name
+    Integer potatoCount
+}